They have to join together. My husband who worked for one of the largest general contractors in the US, said he wouldn’t do it. But call a roofer and ask. He said the problem comes, not so much with it will look really odd, but you will have to mess with the other owner’s roof.

With three tab shingles, when you put shingles on a house, it is like a brick pattern. The starter row is three of the tabs, the row above is cut to start with two of the tabs, the next row above is cut to start with a single tab…on and on…three, two, one. They will be messed up when you cut the other owner’s shingles to put yours on the roof at the center. Plus, you’ll cut the tar paper or felt underneath his as well. My husband said good luck but get a couple of opinions from roofers.

I was a roofer for years and it can be done. If it only has one layer of roofing on it you can apply the new roof right over the existing one ( check your local building codes). However if there is more than one roof on it or if you were planning to tear off the existing 3 tad I would advise against it.